RE: cooperative multitasking

["Curt Carpenter" <curtc@...>]

Yep, Lua's built-in support for 'coroutines' is great for those who
don't mind the "unfortunate but unavoidable restrictions in your C
functions".

-----Original Message-----
From: Jeff Petkau [mailto:jeffp@adrenium.com] 
Sent: Friday, June 07, 2002 2:02 PM
To: Multiple recipients of list
Subject: RE: cooperative multitasking


> > Hmm, in what I've worked on, a thread switch in the script engine 
> >could be as fast as changing a pointer
>
> The limitation is when you have a call into a registered function, and

> you want it to block and return control to your "normal" non-Lua C 
> code. At that point you have a call stack that looks something like 
> this:
>
> MyRegisteredFunction
> luaD_precall
> luaV_execute
> luaD_call
> f_call
> luaD_runprotected
> lua_call
> 
> So with a fiber, all that stays as is, you switch to your C code, and 
> switch back when you like, and when you do that stack is still exactly

> as it was. With a coroutine, if you want to yield 
> _but_not_to_another_Lua_thread_, you have to unwind all that, and wind

> ...etc...

There's a much, much simpler fix for this: Don't Do That, Then.

C and C++ do not support microthreads (I'll follow Edgar's advice and
stop calling them coroutines.) Using Lua as your script language will
not suddenly make C and C++ start supporting microthreads, no matter
what you do. But with the simple, obvious, fast implementation, you can
have excellent microthread support *In Lua*, with some unfortunate but
unavoidable restrictions in your C functions--namely that a C function
called from Lua, which calls back into Lua code which might yield, must
explicitly push its state and use a trampoline. It sounds like a pain,
except that in practice it almost never happens anyway; by going with
fibers you'd be eliminating huge practical benefits in favor of almost
purely theoretical ones.

[Sorry, I often tend to rant for no reason.]

--Jeff